1. Field of the Invention
The present invention relates to an image taking apparatus such as a digital camera, etc., which can obtain an image with an excellent image taking accuracy by correcting image blur caused by vibration.
2. Description of Related Art
In cameras existing presently, operations important for image taking, such as exposure determination and focusing, etc., are all automated, and even a person who is not skilled in camera operations has less possibility of failing in image taking.
Furthermore, recently, a system that suppresses image blur from being caused by vibration applied to the camera has also been studied, and factors that cause a photographer to fail in image taking have been reduced to almost zero.
Herein, a vibration isolation system that suppresses image blur is briefly described.
Vibration applied to a camera when image taking appears as vibration with a frequency of, normally, 1 Hz through 10 Hz, and as a basic consideration for enabling the camera to take an image without image blur even when such vibration occurs during image taking, it is required that camera shake due to hand vibration is detected and a correcting lens is displaced within an optical axis orthogonal plane according to the results of the detection (optical vibration isolation system).
Namely, in order to take an image without image blur even when camera shake occurs, it becomes necessary that, first, camera shake is accurately detected, and second, an optical axis change due to the shake is corrected.
Vibration applied to a camera can be detected by, in principle, mounting on a camera a vibration detecting unit that detects acceleration, angular acceleration, angular velocity, and angular displacement, etc., and carries out proper calculation for the results of the detection. Then, by driving a vibration correcting optical unit (including a correcting lens) that corrects image taking optical axis displacement on the basis of the detection information on camera shake outputted from the vibration detecting unit, image blur correction operation is carried out.
On the other hand, a method can be used in which an image pickup operation is repeated a plurality of times with an exposure period with a length that does not cause image blur, and a plurality of images obtained through these image pickup operations are synthesized while correcting image displacement among the images to obtain a taken image (synthesized image). This method is disclosed in Japanese Patent Publication No. 3110797 (hereinafter, referred to as Document 1).
Recent digital cameras have become smaller in size than silver haloid compact cameras, and in particular, a camera that has an image pickup device of a VGA class has been downsized so that it is built-in a portable electronics device (such as a portable phone).
In these circumstances, in order to mount the abovementioned optical vibration isolation system on a camera, it is necessary that the vibration correcting optical unit is made smaller or the vibration detecting unit is made smaller.
However, in the vibration correcting optical unit, since a correcting lens must be supported and highly accurately driven, there is a limit to downsizing. In addition, most of the vibration detecting units that are currently used utilize inertia, so that if the vibration detecting units are downsized, detection sensitivity lowers and accurate vibration correction becomes impossible.
Furthermore, vibration to be applied to cameras includes angular vibration around a predetermined axis and shifting vibration that shakes a camera parallel, and although the angular vibration is correctable by the optical vibration isolation system, the shifting vibration is hardly corrected. The shifting vibration tends to become greater as the camera becomes smaller.
On the other hand, as a different vibration isolation system, as has been employed in a video camera for taking a moving image, a method in which a motion vector readout from an image pickup device is detected and an image readout position is changed according to the detected motion vector to obtain a moving image without image blur can also be employed.
This method has an advantage in that the camera can be downsized since the vibration detecting unit and the vibration correcting optical unit (including a correcting lens) in the abovementioned optical vibration isolation system become unnecessary.
However, the vibration isolation system used in video cameras cannot be easily applied to digital still cameras. The reason for this is described below.
Motion vector extraction is carried out for each image reading, and for example, when images of 15 frames are extracted per second, motion vectors are detected by comparing these extracted images.
However, in a case where a still image is taken by a digital camera, exposure is carried out only once for the image pickup device, so that detection of motion vectors through comparison of images as in a video camera is not possible.
Therefore, the vibration isolation system for video cameras cannot be simply applied to digital still cameras.
On the other hand, in the vibration isolation method disclosed in Document 1, since an image pickup operation is repeated a plurality of times, an image taking period becomes long.
This long image taking period does not pose a problem when an image of a still object is taken. However, when an image of an object such as a person that slightly moves is taken, shake of the object side (object vibration) is caused, and image deterioration caused by object vibration cannot be suppressed although image blur caused by camera shake can be suppressed.
Furthermore, as in the case of a video camera, if composition displacement is always corrected based on a motion vector, a taken image is left locked on an object at an initial point of image taking even after the camera is intentionally moved for panning, and this makes it impossible to obtain an image which a photographer intends to take by framing operation (determining an image taking composition).
Therefore, usability of the video camera is improved by intentionally attenuating low-frequency components of motion vector signals (displacement signals). This results in lowering in vibration isolation-accuracy, so that even by synthesizing a plurality of obtained images, it is not possible to obtain an image whose blur has been highly accurately corrected.